The present invention relates to the field of aircraft engines, for example turbojets, and it relates more particularly to a casing structure interposed between the engine and the nacelle.
In known manner, in addition to providing the mechanical junction between the engine and the nacelle, such a structure must:                provide continuity of the aerodynamic passage for the bypass stream;        pass and hold (electrical, mechanical, hydraulic) services between the various components of the engine (core, fan, etc.) and the nacelle of the engine;        provide fireproofing protection between the various compartments of the engine and the bypass stream; and        provide accessibility to pieces of equipment and to services for maintenance purposes.        
The casing structure is made up both of a sectorized cylindrical frame (or of shroud sectors) of small width (of the order of a few hundreds of millimeters around the axis of the engine) suitable for giving easy access on opening the nacelle to the pieces of equipment that it covers (e.g. the thrust take-up rods, or indeed various actuators for actuating services), and also of at least one radial arm arranged around the frame. Conventionally, the frame also supports scoops of the air bleed system (variable bleed valve (VBV) booster).
Since the time required for maintenance needs to be optimized, the frame thus constitutes an obstacle to accessing pieces of equipment placed beneath it, to enable them to be removed, changed, or verified . . . , on apron and on opening the nacelle in time that is limited and depends on the application, and that needs to take account of the time required for removing and replacing the frame itself.
There thus exists a need for a casing structure that provides easy access to these pieces of equipment (i.e. access that is compatible with the time authorized for maintenance on the ground), while enabling all of the above-mentioned functions to be performed and in particular while ensuring reduced disturbance in the air flow passage.